1. Field of the Invention
The present invention relates to optical sources, and more specifically, it relates to enhanced scaling of the pulse energy and brightness of optical sources.
2. Description of Related Art
The need for, and, scalability of, high-energy, high-average power optical sources continues to grow with increasing demands in such diverse fields, including materials processing, precision drilling and machining, homeland security, energy needs, defense, oil-well exploration, remote sensors, medical technology, 3-D lithography and nanotechnology.
Practical and fundamental issues necessitate the need for novel approaches to the design and realization of next-generation optical sources. Practical issues include efficiency, size, weight and volume constraints, whereas fundamental issues include optical damage limitations and thermal and nonlinear optical effects, to name a few. Some of these constraints limit the design of the laser itself, whereas other constraints present engineering challenges to the realization of methods to, in effect, combine the outputs of lasers and optical amplifiers, which are otherwise limited as stand-alone devices.
Typically, in addition to high-power and high-energy, lasers with high-quality output beams are in demand to meet the expectations for precision processing, high-brightness applications and overall system efficiency. As an example, in many applications, a laser system that includes a coherent summation of high-quality lasers and/or laser amplifiers can provide a viable solution, yet, in general, is fraught with its own unique limitations.
The last decade has seen a tremendous advance in the pulse energy output of double-clad 1 μm fiber amplifiers. As these devices reach their single aperture damage-limited pulse energy, there is renewed interest in beam combination as a means of scaling the energy further. Note that prior art that employs an optical fiber for Raman amplification is not applicable to solving the issue of attaining >4 MW through beam combination, as the 4 MW limit is imposed by the fiber itself.
Maintaining good beam quality remains a challenge in fiber laser systems that are designed to generate millijoule class, 1 ns pulses at high repetition rates. In the prior art, the optimal approach involves design rules that conflict with one another. As an example, to achieve high-pulse-energy output from fibers, the core diameter needs to be increased to stay below the damage fluence, while the numerical aperture (NA) is reduced to ensure that only a few transverse modes are guided. On the other hand, to achieve a compact geometry, the fibers need to be bent, leading to increased radiative losses for the higher-order modes and reduction in the mode field diameter and, therefore, the extractable pulse energy. Pulse energy scaling of a single-aperture fiber laser or amplifier with good beam quality is also difficult, because long fiber lengths combined with tightly confined modes and high peak powers can trigger the onset of undesirable nonlinear effects including stimulated Brillouin scattering and stimulated Raman scattering, thereby limiting the energy scaling.
Experimental results showing significant brightness enhancement and 60-70% conversion efficiency were demonstrated by the LNL group led by Goldhar and by other groups as well. With the rapid development of bulk diode-pumped 1 μm solid state lasers and UV conversion nonlinear crystals since the late 1980's, some of the interest in Raman beam combining waned, with relatively fewer papers being published since the 1990's [Heuvel1992, Heuvel1993, Heuvel1995, Murray1999, Chulkov2006].
Beam combining approaches discussed in the literature fall into two broad categories: incoherent and coherent. In the case of incoherent beam combining, multiple free-running, uncoupled and independent input lasers are utilized. Each laser operates at nominally the same or slightly different wavelengths, with the ensemble positioned side-by-side and allowed to combine in the far field. Given that the relative phases or spectra of the laser elements are not controlled, the resultant radiance or brightness (B=P/(λ2(M2)2)) of the combined beam is therefore not any greater than that of a single laser. If the wavelengths of the individual laser elements are different and carefully chosen, it is then possible to use a dispersive element, such as a grating, to combine the individual laser elements in the far field. This incoherent approach is called “wavelength beam combining” (WBC) and has been pursued by the Lincoln Lab group which has achieved 35 W with M2=1.35 in both dimensions from 100 laser diode elements [Chann2005]. With WBC, the individual elements overlap both in the near and far field, and, consequently, the spatial brightness therefore scales as a function of the number of individual elements, N. Since the power spectra of individual elements in the WBC approach are not allowed to overlap, spatial brightness is enhanced at the expense of spectral brightness. In a key end-user application of interest, this approach is difficult to practically implement, because the required output line-width, in this case, must be less than 50 GHz.
The other major approach to beam combining is referred to as coherent beam combining (CBC). In the simplest configuration, the multiple lasers are, as before, positioned side-by-side, but, in this case, fabricated to operate at the same center wavelength and spectrum. In addition, the ensemble of laser sources are phase locked so that their fields add coherently in the far field. If the phases are controlled to within a small fraction of the wavelength, the total power and brightness scale proportionally to the number of individual laser elements. Over the past three decades, several different implementations of CBC have evolved. In one case, referred to as the “common-resonator” approach, typically, individual laser diode elements are placed inside a common optical resonator. Coherent beam combination takes place due to feedback from the resonator [Leger1987, Leger1988, Corcoran1991, Kono2000, Fan2005]. In another case, referred to as the “evanescent-wave” or “leaky-wave” coupling approach [Welch1994, Fan2005], semiconductor laser elements are placed, in close proximity to each other, so that their field distributions overlap. If the neighboring elements are in phase, then it is possible to achieve high on-axis far field intensity. This approach, however, is difficult to achieve in practice with large arrays because the some of the elements tend to combine π out of phase.
Yet another CBC approach, [Ishaaya2004, Minden2004, Fan2005, Corcoran2008, Bochove2009] which has been specifically applied to combining fiber elements is referred to as the “self-organizing” or “supermode” approach. In this approach, multiple fiber lasers are placed in a common resonator. The spectrum of each individual fiber laser self-adjusts, as the elements of the ensemble injection lock to each other and minimize the loss of the array. Approximately 10 fiber lasers have been combined with this approach, though further scaling has been difficult to achieve.
Still another major CBC implementation approach uses active feedback to control the phase of each individual element. This approach has been mainly used recently in the context of an array of fiber amplifiers (as opposed to laser oscillators) configured in a master-oscillator, power-amplifier (MOPA) arrangement, which is seeded by a common (i.e., master) oscillator. The wavelength-scale (modulo 2π) path-length differences amongst the fiber array amplifier elements are detected by heterodyne mixing of the output with a reference laser. The generated feedback is used to drive modulators that add the appropriate amount of phase to each individual laser amplifier to phase-lock the ensemble. As a result of these parallel, servo-controlled amplifier legs, the resultant path-length differences are nulled out, with the result that the waves from each element constructively interfere to produce an enhanced brightness output in the far field. [Anderegg2003, Augst2004, Fan2005, Shay2006].
In addition to the linear approaches described above, the prior art also includes nonlinear optical approaches to coherently combine laser elements. This technique is referred to as “optical phase conjugation.” and uses a nonlinear optical interaction in a given medium to realize phase coherency amongst an ensemble of laser amplifiers. Examples of such optical interactions for coherent beam combining include stimulated Brillouin scattering (SBS) [Moyer1988, Rockwell1993, Sumida1994, Fan2005] and stimulated Raman scattering (SRS). In a key end-user application, the optical pumping laser elements are necessarily broadband spectrally. Given the constraints of SBS, the use of broadband pump sources precludes the use of this nonlinear optical interaction for efficient beam combining. Hence, phase conjugation via SBS is therefore not an option.
Stimulated Raman Scattering has been studied over the last three decades. Much of the early work in the late 1970s and 1980s has been in the context of improving the brightness of UV generating excimer lasers [Goldhar1982, Chang1983, Goldhar1984, Chang1985, George1985, Eggleston1986JOSAB, Eggleston1986JQE, Korff1986, Komine1986, Reintjes1986, Fulghum1986, Partanen1986, Shaw1986, Flusberg1987, White1990]. These papers elucidate the theory of Raman amplification systems with either single or multiple coherent and incoherent pump sources, narrow or broad bandwidth pumps sources, and collinear or non-collinear interaction between the pump and signal waves. These papers also model the beam-quality of the amplified Stokes wave with moderate to severe input pump wave aberration. Experimental results showing significant brightness enhancement and 60-70% conversion efficiency were demonstrated by the LLNL group led by Goldhar and by other groups as well.
As noted above, with the rapid development of bulk diode-pumped 1 μm solid state lasers and UV conversion nonlinear crystals since the late 1980's, some of the interest in Raman beam combining waned, with relatively fewer papers being published since the 1990's [Heuvel1992, Heuvel1993, Heuvel1995, Murray1999, Chulkov2006]. The last decade has seen a tremendous advance in the pulse energy output of double-clad 1 μm fiber amplifiers. As these devices reach their single-aperture damage-limited pulse energy, there is renewed interest in beam combination as a means of scaling the energy further. Since these pulsed fiber amplifiers possess relatively high peak powers, it makes sense to re-employ the Raman effect, which is peak-power dependent, to enhance the brightness and peak power available from a single aperture.